Facsimile transmitting system



March 31, 1953 4 M. DQMFARLANE 2,633,494

' FACSIMILE TRANSMITTING SYSTEM 7 Filed March 2:5, 1946 Patented Mar. 31, 1953 UNITED STATES EPATENT OFFICE This invention relates to improvements in facsimile transmitting systems, and, in particular, to the control of transmitted signals in phase and time relationship in accordance with the tone of the subject copy to be transmitted. This invention in the preferred embodiment here described and illustrated relates; to the transmission of black-and-white original subject copies. 1

In methods of facsimile transmission used heretofore, particularly methods employing a sub-carrier signal for the transmission of the facsimile intelligence, an alternating current signal of a specified frequency or within a specifled range of frequencies is used as the sub-' carrier signal for the transmission of the intelligence. This frequency may be generated, for example, by a chopper actin through the optical system, or it may be generated by an electronic oscillator or other device suitable fo'r the pro duction of a train of oscillating waves. In systerns employing an optical chopper, the speed of this chopper is normally constant, and this speed may bear a definite relationship to the rate of scansion, orit may be independent of this rate. In previously knownsystems, the optical chopper is inserted in the light pathbetween the light source and the photoelectric cell and is continuously operating: it is so arranged that a variation in the shading or the original subject copy produces modulation of the intensity of the light beam, each signal at the chopper-controlled fre- Eiuency being of an'amplitude corresponding to the tone shading of an individual unit area of the original subject copy. The resultant photoelectric cell output is an alternating current signal of frequency controlled by the chopper and of amplitude controlled by the intensity of the shading of the subject copy at each individual point of scansion, this'is, the shading of each unit area of the original subject copy -Where the alternating signal, which may be of the sub-carrier variety, is provided from an oscillatory source such as a vacuum tube oscillator, the usual arrangement heretofore has been the provision of a modulating element (usually a vacuum tube or tubes) whereby a suitable signal .(WhiCh may be in the form of an undulating direct current signal) from the photoelectric cell .or its. associated circuit elements is caused to f modulate the amplitude or frequency of the oscillatory signal so that the resulting output is a signal which is modulated in respect to amplitude or frequency by electric currents corresponding to the tone shading of individual areas of the original subject copy. v

When these existing systems, either of the chopper or modulator type, are used for the transmission of original copy which is black and white only, and which comprises no intermediate tone shadings, false tone signals may occur *at the'start and at the conclusion of a black segment on a white background or white segment on a black background or the original subject copy. Due to the finite aperture size necessarily employed, the light intensity striking the. photoelectric cell as the scanning proceeds from black to white or white to black is translated gradually from a minimum to a maximum or maximum to minimum, as the case maybe. As these translations are superimposed upon either the chopper frequency or the oscillatory frequency', there results a random relationship between the individual cycles of the said frequencies and the modulating signal which correspondsto the variation in tone shading occasioned by the transfer of tone value. In a system which is adapted to transmit and record tone shadings, this resuits in an intermediate signal value being transmitted over the transmission circuit at such times as this effect occurs, and in the reproduction on a variable density reproducer of a tone of intermediate value between black and white corresponding to the intermediate value of the transmitted signal; 3 j

I-Iowever, black-and-white transmissions. may be achieved over *circuitswhich are not susceptible to the transmission of intermediate .tone

shadings. The'elimination of the intermediate tone shading for black-and-white transmissions increases the efficiency of the circuit and permits the handling of a greater amount ofcopy ina given time, With on oii transmissions, as may be employed in such black-and-white transmissions, circuits have inthe past been employed to eliminatethe signals corresponding to intermediatetone shadings. These circuits may comprise acombination of limitors, square wave networks, and other signal shaping devices. The

resultant output from these devices is a train of waves of constant amplitude and predetermined frequency whose start is controlled by the combined effect of any shaping network that may have been employed and the random relationship of the phase ofthe chopper or oscillatory frequency to the true arrival time of the white-toblack or black-to-white transition before the scanning mechanism. This means that the recorded signal will start in some spatial relationship in respect to its correct position, the displacement of which depends upon the phase angle above discussed.

One feature of this invention, therefore, is dirooted to correcting this displacement. In a device constructed in accordance with a preferred embodiment of my invention, the signal frequency, be it sub-carrier or carrier, is initiated by the transition from black-to-white or white-toblack, as the case may be. While this may result in a slight time displacement between the transmitter and receiver signal positions, this displacement; is uniform throughout the whole transmission and results in a more-correct rendering of the physical outlines of the black-and- White original subject copy than is possible by means in prior usage. The desired initiation-of the transmitted signal may be effected by so controlling the chopper that itsoperation is. interrupted and related to the transition ,inblack and White scansion of the original subject copy, or alternatively, in the case of an osci11ator,;.'this may be efiected by shock-excitation of the osic at rint pe ati y;the-transition.ill bl aa wh t an n, i he s w th .onwhhcut .the interposition ofany shapingnetworks. By a, cor- 1 me cont o 91; th circu t it-is .possit1e, to. cis- ..rcns w t sha in networks as h r tofore u d,-

T and ;one aspect. of.tl1e. i nvent ion is. directed to .izhatend.

:rlire iou z has be n proposed; tocontlffll t e initiation of a..to r 1e lsi nallwithin the; space of time allotted; for the transmission .of ;a signal cor- :resccndimfimauni -area o the,.,c.rie na1 subiec ,copy, sothatthetone value f.such;a .unit area is term ed; o e y baa-time ementwithin the -time-otthe transmission .of .aunitarea, as. defined t *myfia ent-No.3, 79 1 dateiF uary 6, al.9 2 Th s-sis t h lea ly disti ui ed. from :rthezccntmlrof-the. nit a on .of a train of -w s Lin ac ordan zwit. th nresent inv nt on; said. str i :bfiil ght fie i itiatedrnot,w thinthescansicn :tirc l l i area,-.bu :-.ac ua lv to ,clete mine nthenni area: d sposi on. rr spe t ve of t ne val- ;u ssex cn i n-ch nces om lack to wh te a d swhiteqt wh ac cfithe nrieinaL- icct c py h t ic previously nis lq edre ates t t meshes ent :With nanrcde erm nedim a e patte whilat einethod her :d sclo edcon ols thau t -;area pattern in accordance withcthetransitions of thesoriginal: subject, copy. According tothe embodiment of the invention presented here,

-control is :exerted i'IEOI blacksand white subject r.

-matter and not -for subject material having tone .-.varia-tions- :therein.

Another object of my invention' is to dispense --with the-transmission ofsignals other than those representing values of toneof" the original subject-copy in the blackand white category. Itis .clear that these signals may be sub-carrier signals to be subsequently modulated upon'a carrier -circuit,-or theymaythemselves-be carrier signals ofthe -requisite'irequency-and therefore be transm-itted-directly: an object ofthis invention is therefore to secure this arrangement of signals, either as carrier or sub-carrier frequencies for transmission.

5By-the elimlnatlon of-intermediate-tone signals,

thelbandwidth necessary for the transmission of facsimile intelligence is very considerablynarrowed. Qne object of my/invention is to take advantage of-this narrowed band width by econmizing in'linetransmissiontime, or, alternativelyybyincreasing the amount of subject detail transmitted ina giventransmission'time. 7 Other objects of my invention will be evident from inspectionof the following 'detailedspecification and of the drawings annexedhereto, which drawings are appended for purposes of illustration only, and which embody certain preferred forms of the invention but do not limit the scope thereof.

In the drawings:

Figure 1 is a schematic illustration of a facd. subject copy transmittedin accordance with my invention.

Figure 4 is an illustration of a chopper control :means for carrying out the purposes of this inmention.

Figure?) is a clock diagram of a signal gen- ..-.erator-circuit: operating in accordance with this invention.

"Figure 6 shows a schematic diagram of a con- ;trolled transitron circuit adaptable to the purposes of this invention.

Figure 7 illustrates aschexnatlc (diagram of .another. formoftransitron circuit.

Figure 8. is ,a. schematic representation. ofa ShOCkBXCl'GQCL oscillator. employed 1 according to this invention.

Figure ,1. illustrates. a transmittingmechanism at the, left and a-receivingmechanism at the right.

,HAt-the transmitter .acylinderl is driven bya motor 2 through suitable gearing (not shown). The speedofthe motor; 2;.may be controlled by a. motor control ,nnit. 4. -..Gearing-5, between the drum iv. and.. the,lead screw 6; controls'the-trana lensing. 0f the? scanning ,unit 1, whichrmay comprise ,a light source and photoelectric cell in operative placementgand provided with, asuitable optical system. .flheelectric signal from thapho- .,t,oelectr ic cellin the .unit .7. is passed tothe signal r-gcneratonfi and thence to the amplifier v 3 ,-which amplifier may ;be of any 1 suitable type adapted to put the signal -upon a-wire lineorhponfaradio transmittin acircuit. flfhedashedlines vlll'repiesent the communication channelythismaybe a wireline, but, it isitogbe understood that.- my in- :vention is notlimited tothe useof a wire linefor 1signal transmission'between ,the transmitter and receiver, a and that-thecircuit Hi may bereplaced by: r dio. link. or- ;oth.er; suitable communication means without departurefrom the spirit-of;my intention.

-;At ther ceirinastation the. drum Misfiriven :bythe; motor L2 which may.becontrolled-:bwthe :m tcrcontrohunit. i 4- :This l ot l'icontrollumt u i 42st the stransm iter, :or. i may: be owind pe d nt control. if sired, :signals ma be rtia-nsmitted overgthegcircuittl 9 which will; control the: speedand/ or the phase relationship of the transmitting andsending drums l and Why a :suitable control exercised upon the motor control unit t4. Theincoming signalsfrom the line I!) may be amplified bythe amplifier Wand-passed to the-control unit it, inwhichthey may be recitified'or otherwise rendered suitable for-thecper- -ation of the recording device N. This recording device is caused by means of the lead screw it andgearing t5 toscan the surface of the receiver the-gearing '5 transverses the scanning unit! 7 over the surface of the original subject copy which hasbeen placed upon the drum I. The

'pl'ete signal. mechanical arrangements of the receiver and the.

photoelectric cell output from the unit 1-is amplified (if required) and is caused to operate one of the transitron or shock-excited circuits describedbelow, and which is in the signal gen erator unit 8." The output from the signal gen-' erator unit 8 is therefore a series of wave trains of full amplitude, whose start and finish correspond to the placement of the transition between black and white of the original subject copy on the drum' I as scanned by the unit I. After transmission and amplification by the amplifier l 9, (if required), the received signal trains pass to the control unit l8, where they may be rectified, doubled, or otherwise treated in any suitable manner to effect the recording through 1 the agency of the unit II. If this unit I1 is of the galvanometer type, A. C. signals may be used thereinyor the signals may be rectified and the envelope applied to the g-alvanometer. A recording glow lamp may be used in the device [1, incl;

which case the control unit i8 may feed out the desired signal which may be in. the form of rec-i tified pulses ormay be an envelope of the com-I Due to the similarity between the transmitter and to the motor control which is exercised for maintenance of speed and'for correct phase, the unit I! is caused to scan the record-;

ing surface of the cylinder I i in a corresponding manner to the scansion of the unit I over the surface of the cylinder i at the transmitter.

In Figure 215 illustrated a line, a facsimile of which is to be transmitted. This line is so arranged in relation to the chopper or oscillator frequency and the scanning speed that a succession of oscillator signals appear with a front parallel to the portion of the line to the right of the bend. In this figure, for purposes of illustration only, it is to be considered that the line is black although only the end portions thereof are so marked. To delineate the boundaries of this black line, the lines 3|, 32, 33, and 34 have been" drawn, it being understood that the area enclosed in these four lines is scanned as black by the optical unit. The thickness of the line illustrated in Figure 2 is approximately the width of one unit area; i. e.'; for correct reproduction, accurate transmission and placement of a single unit of the carrier or sub-carrier frequency as desired. As shown, therefore, a single unit of the type illustrated as 35' represents this line upon the reproducingsurface.

' Examination of Figure 2 discloses that there is a discrepancy (due to a time lag between the placement of the unit areas 35a. and 35b, for example, and the leading edge of the line illustrated by the outline 3|. This displacement as described above is due to the fact that the signal generator in this particular case is assumed to. be out of phase with the transition signal, and that therefore'the recorded signal cannot be 'placeduntil a peak of the signal generator signal signals at points where the edge of each .step .;.over1aps.the edge of the recorded line.

It is to -be noted that in-this figure an illustration has been chosen in which there is a fixed relationship between the frequency of the oscillator or other device producing the signals 35 and the speed of rotation of the drum l at the transmitter so that'the front presented by. the adjacent signals is parallel to the line 3|. This is not necessarily the case in all equipments, and various and changing relationships may in practice exist. Where these exist, other configuraions than that shown in Figure 2 will result-but in general the situation will-be similar as there will be some relationship between the scanning factors and the angle of the line which will produce a result of the type shown in Figure 2.

In Figure 3 is shown a similar bent line to that shown in Figure 2. Also shown in Figure-3 are unit recorded areas 45a, 45b, 45c, 45d, 45e; 45f, and 45g. The recording effected in Figure 3, however, is carried out by means of a start-stop signal generator as has been above described, and

' as is illustrated in some of the other figu'resherein. A feature of this start-stop signal generator is that the train of signals corresponding toany particular transition from black to white or from white to black is started in a definite phase relationship to that transition. Thus the leading edges of the signals 450 151), 45c'and 45d coincide with the leading edge of the line,'and the signals 45c, 45] and 459 are displaced a'small amount only in order that the condition of transition may be met. In other words, the-displacement of one unit, for example 45a from' 45d is not the displacement of a complete cycle or halfcycle of the sub-carrier or carrier-frequency, but is'only such portion of a cycle as is represented by the time difference in the transition due to the slope of the line at that point.

Examination of Figure 3 will further show that under the circumstances illustrated no double signal is produced, and that the stepped increments due to the slope of the line are smaller than those of the correspondingportion of Figure 2. The resultant effect of Figure 3 is a more faithful reproduction of both the horizontal and inclined portions of the line.

It is to be noted that in both Figures 2 and the effect discussed is produced by asign-al on black. It is pointed out thata si 'n'il-ar result is. obtainable by inversion where a signal on white is required and a whitellline is to herepro- Iduced. The same principles illustrated in Figures 2 and 3 can thusbe'appliedt o an-yt'rans'itionfrom black to white or white to black. J In Figure 4 is shown astart-stop'signalgenerator of a type in'which a rotating chopper 1 mechanism may be employed according to this invention, It is pointed out that the device illustrated in Figure 4 is for purposes of illustration only, and that the inventionis not limited to the actual construction there shown,

.In Figure 4 a light source 5! is imaged onto the surface of the original subject copy 52 by means of lenses54 and 55. The optical system comprising these lenses 54 and 55 may be so arranged that there is an optical crossing point between the. lenses, and at this point. the chopper disc 56 is arrangedto cut the beam. The disc 55 is shown in this illustration as an -edge view, but it may comprise a circular disc having therein slots or holes of the type well known in the art; A suitable form of chopper disk is shown in my Patent No. 2,16 l,038, dated June 27.; .9 e 5fi ctes f om e su faced acetate the original easiest copy -2 is directed to the peritoneum can 5 The disc 56 is adapted to be driven by the sister 58, whichlis itself controlled by the moerr t'ontrol 5-9. The drive between the motor 55 and the disc 55 may be through the magnetic icl utch 68. Also attached to the disc 56 is the "serrated clutch 6'! adapted for engagement with the stationary serration of the unit 52 on the arm 64. Under normal operating conditions, with a white surface of the original subject 52 'beiiig scanned; the member 62 is engaged with the surface of SI by means of the pivoted arm -64 and a spring '65 co-acting to secure engagement. The serrations of the member 'BI are so located relation'to the apertures in the disc '56 that the latter will be stopped with "an apertore-iii the path of the light beam.

electromagn't E6 is arranged adjacent to the-arm 64 and is adapted to release the unit 62 from engagement with the member Bl against the action of the spring '55-. As shown in this embodiment of this device, the electromagnet 6B is energized at the same time as the magnetic clutch 60 is energized. This may be performed through the clutch amplifier 57, which is controlled by the control unit '68 which is so coupled to the photoelectric cell unit 57 that it will cause the clutch amplifier 81 to operate the magnetic -clutch 50 and the releasing electromagnet 68 whenever a transition occurs at the surface of th'eoriginal subject copy 52, i. e., when the tone changes from white to black. 1 The motor 58 continues to drive the chopper 56 while the black material on the original sub- --ject copy 52 is being scanned. 'As soon as another transition occurs and the copy 52 becomes white, the output of the photoelectric cell unit 51 will rise, the control unit 68 will reduce the output of the clutch amplifier 61, which will release the magnetic clutch 50 and the magnet 56. Release of the clutch BB and magnet 66 allows the member 62 to engage the unit 6| under the influence of spring 65, resulting in "stoppage of the disc 56 with an aperture in the path of the light beam. The cycle of operations may then be repeated. The line amplifier 69 transmits to a line or to a radio or other communication circuit impulses generated in the photoelectric cell unit 515. It is to be noted that during the traverse of a white section befdi ie the optical beain there will be no altervhating signal transmitted by the line amplifier because the disc 56 is then held stationary and the output of photoelectric cell is 'a direct current. when there i's' a black section or the origisubject copy in the {path or the light seem. an alternating current signal will be generated through the action of the chopper disc 56 and this signal may be transmitted to the line by the line -airlplifi r '69. Alternatively, the unit 5 1 may be arranged in such a fashion that a stron fsiglla'l "will be delivered While black is bein scanned, anda weak signal on white. The other circuits may then be arranged accordin ly.

While a rem-ting chopper form or light interroster has been here illustrated, it is pointed out that an ei'iactly analogous arrangement may be effected by the use or a vibrating chopper device, that is lone in 'which the light beam is interrupted by a ot'raung memuer which may be"electromagneticallycontrolled. Also the same effect may be secured by the. use of a Kerr'cell, use ct'giiti'ol of this (cell being however electrical. Tire-Kerr cell adaptation may-use part porinput circuit 81.

8 sons or the device's according to both Figure =4 and Figure 5.

Figure 5 illustrates an arrangement of an optical system and start-stop signal generator in which electronic circuits are used in place of the chopper device illustrated in Figure 4. Light from a source H is focussed by means of a lens 14 onto the surface 12 of the original subject copy. Reflected light from this surface is picked u by the photoelectric 0611 H which 'is adapted to control the start-stop signal generator 18 in the production of trains of oscillatory waves'c'o'rresponding 'to the space between the transitions from black-to-white=to-black, or white-to blackto-white of the original -subject copy 52. The output from the signal generator '18 may be amplified by the line amplifier 19 and thereby transmitted to the line or over a radio circuit or other communication channel as may be required.

Figure 6 illustrates a transitrcn oscillator which may be triggered by 'a negative signal to produce a train of waves. The tube 81 (here shown as a pentode) is supplied with a negative signal on its suppressor grid through the The negative signal on this grid causes the tube 8! to oscillate at a frequency determined by the characteristics of the tuned circuit comprising the inductance 82 and the capacitor 84, which are connected across the suppressor and screen grids of the tube. The resultant oscillations appear at the output 88, and as these oscillations are triggered by the negative signal at the input 87, they are maintained for as long a period as the input signal remains negative. I

From examination of the circuit of Figure 6 it may be seen that the peak of the first half-cycle tron device will occur at an exact period of the time after the input signal reaches-a predetermined negative value, so that it is established that a signal will be transmitted to the line or radio circuit or other communication channel-at a precise time after the occurrence of a tone transition from black to white or white to black,

- as the case may be. Also-this time interval will be the same for each similar transition. By proper circuit design the time lag occasioned may be rendered negligible, or may be compensated by a suitable adjustment of the phasin mechanism since the time interval is the same in the case of each transition.

Figure '2 illustrates a similar circuit in which the tube 3| (also arranged as a transitron oscillater) is controlled bymeans of the switchingtube 83. A signal in the input circuit 91 causes the tube 93 to change its state, and by its abrupt change of state this tube throws the transitron oscillator 9! into a train-of oscillations whose frequency is determined by the-combination-of the inductance '92 and the capacitor 9d. The output circuit 98 carries a train of waves of the required frequency whose phase with respect to the incoming signal in the circuit 97, and therefore'with theB positive supply. The application of a nega- Etlve signal to the input circuit causes the grid lil5 of the tube IM to drive the tube to cut off. The oscillatory circuit comprising the inductance I02 and capacitor I04 arethrown into oscillation at a frequency determined by these components, resultingin the output of a signal of the desired frequency across the terminals "it. In theparticular circuit shown here, the signal in the circuit I08 will die .away due to the decrement of the oscillating circuit; if a continued signal is required, however, a sustaining amplifier may be built with the circuit elements 162 and HM therein so that a train of oscillation is sustained after having been produced as abovedescribed.

' In the above figuresthe input circuit has been illustrated as taken irom'a photoelectric cell. "It is to be understood that this is not necessarily the connection that is to be employed; the photoelectric cell output may be used directly as shown or it may be amplified with a suitable amplifier and may, if desired, be subsequently rectified if the amplifier output is of an alternating character. For example, the photoelectric cell shown diagrammatically in Figure 5 at H may in fact comprise a 93l-a. electron multiplier phototube in an oscillating circuit at 160 kc. or better, the output being suitably rectified to provide the control signals for the transitron or shock excited oscillator shown elsewhere. For transmission over a normal telephone line, for example, the frequency of the transitron oscillator may be in the neighborhood of 2,000 cycles per second; hence the error introduced by the time displacement of the 100 kc. signal in reference to the 2 kc. signal is in the order of 2% and is, for practical purposes, negligible.

In the above figures I have illustrated certain forms of oscillatory circuits or of mechanisms which are adapted to the purpose of carrying into efiec't the objects of this invention. However, it is to be understood that the scope of the invention is in no manner limited to the particular forms here shown. The invention is concerned with'a certain relationship between the phase or time relation of a carrier or subcarrier or similar frequency and the scanning transition of black to white or white to black portions of an original subject copy which is to be transmitted; the devices shown here are illustrative of mechanisms and circuits which may be employed to produce the desired results, but my invention is in no manner circumscribed by the particular embodiments and modifications here shown by way of example and for the purpose of explaining the operation of the invention.

' Where the above specification and in the .appended claims I hav referred to a transition it is to be understood that I mean a result of the scansion of a portion of an original subject copy by a beam of light or other suitable means, and

in particular that part of the said scansion which comprises the transition during the process from a white part of the original subject copy to a black portion thereof, or the transition during scansion from a black to a white portion of the original subject copy which is to be transmitted.

And where I have referred to the transmission and/or reception of black-and-white material, and to black-and-white original subject copy it is to be understood that I include (for the purposes of this application) material having a sharp tone boundary such as might exist between contrasting shades of gray, for example, as well as the contrast between pure black and pure white 10 (which shades are in practice dimcult to obtain). Also the term black-and-white as used here would include contrasting material :of othertypeaas may be constituted .by a contrast between shades of sepia, for example, orbetween'shades of any color, or between two or more colors. invention is directed to control of transmission by using a transition from one shade to another .invention is not limited to the use of black-andwhite material as such,

Having now described my invention and illustrated certain embodiments thereof, I claim:

1. In a device of the class described, means to scan the surface of an original subject copy having sharp tone difierentiation and to produce a transition signal therefrom, said scanning means having an optical system including an interrupter adapted to interrupt a beam of light in said optical system in such a manner as to produce a succession of equally spaced light impulses, means to render the interrupter inoperative, and means under the control of said transition signal to render the interrupter operative for interruption of the optical beam.

2. The method of transmitting a facsimile of an original subject copy which comprises scanning said copy by a light beam and producing signals as a result of said scansion, interrupting said light beam at a predetermined frequency, and controlling the interruption of said beam by the signals produced by the scanning process.

3. The method of producing signals for facsimile transmission which comprises passing a beam of light through an interrupter, producing a signal as a result of the scansion of the original subject copy by the light beam, and causing said signal to control the action of the interrupter in accordance with the tone shadings of said copy.

4. In a device of the class described, a iascimile scanner adapted to produce a signal corresponding to a change in tone value of the original subject under scansion, means for producing a suc cession of impulses, and means cooperating with said scanner and said producing means adapted to control the latter to produce a constant time interval between the occurrence of the scanning signal and the peak value of the first of the succession of impulses.

5. In a device of the class described, a scanning mechanism adapted to produce an electrical impulse at a time dependent upon the time at which a predetermined tone difierence is scanned, means for producing an oscillatory signal, and control means for said producing means adapted to cause the latter to produce a first pulse having a predetermined time relationship to the time of the scansion signal.

MAYNARD D. MCFA'RLANE.

(References on following page) Since my 

